Fluoropolymers are characterized by the fact that they are highly inert, paraffinic thermoplastic polymers that have all or some of all of the hydrogen replaced with fluorine. Fluoropolymers include polytetrafluoroethylene (PTFE), fluorinated ethylene propylene. (FEP), and perfluoroalkoxytetrafluoroethylene (PFA), which are all capable of being extruded, stretched and sintered. Much of the work on development of porous fluoropolymer materials, however, has involved tetrafluoroethylene polymers, and processes for producing porous tetrafluoroethylene polymer materials have been disclosed in many U.S. patents.
Porous tetrafluoroethylene polymer products can be produced by stretching an extruded sample of a highly crystalline tetrafluoroethylene polymer resin and then sintering the extrudate while holding it in the stretched state. A dispersion of a tetrafluoroethylene polymer is paste-formed, mixed with a lubricant and extruded. The lubricant is then removed and the resulting extrudate is stretched at a high rate, usually at an elevated temperature less than the crystalline melting point of the tetrafluoroethylene polymer resin. While being held in the stretched state, the tetrafluoroethylene extrudate is sintered by then heating the stretched extrudate above the crystalline melting point. This process produces a material having a microstructure comprising of nodes interconnected by very small fibrils. This microstructure greatly increases the tensile strength of the tetrafluoroethylene polymer extrudate. Because of the node and fibril structure, the material is also substantially more porous than the original extrudate.
The temperature and particularly the rate of stretching greatly affect the porosity and tensile strength of the resulting material. Stretching performed at very high rates produces an increase in the strength of the resulting material. When the unsintered extrudate is stretched at lower rates, limited stretching occurs before fracture of the material, and any materials produced from stretching at the lower rates have coarse microstructures and are mechanically weak. Also, extrudates expanded at both high temperatures and high rates have a more homogeneous structure and a greater tensile strength than extrudates expanded at lower temperatures and lower rates. Therefore, high stretch rates are believed necessary to produce strong materials and both high stretch rates and high temperatures have been recommended to achieve high stretch ratios, homogeneous structures and strong materials.
Furthermore, the primary requisite of a suitable tetrafluoroethylene polymer resin for the process described above is a very high degree of crystallinity, preferably in the range of 98% or above, and correspondingly low amorphous content. Copolymers of tetrafluoroethylene which have defects in the crystalline structure that introduce a higher amorphous content do not work well in the process as homopolymers.
The process discussed above does not generally produce PTFE materials having fine pores less than 2,000 A in diameter. The process, however, can be modified to produce a PFTE material having such fine pores by first stretching the extrudate as discussed above, by then "free" sintering the extrudate by heating it above its crystalline melting point without subjecting the extrudate to tension by holding it in its stretched state, and by then stretching the extrudate a second time at a temperature below the crystalline melting point. The second stretching produces a PTFE material having uniform fine pores between 100 to 1500 A in diameter.
PTFE resin tubes having small pore size and also high porosity can be produced by drawing a tubular PTFE extrudate in the lengthwise direction through a metal die and plug to perform the stretching operation. The thickness of the tube can be reduced to a level not previously possible by radially expanding the tube while simultaneously performing the sintering operation.
The key element of the processes described above is taught to be rapid stretching of the tetrafluoroethylene polymer extrudate. Rapid stretching allows the unsintered extrudate to be stretched much farther than had previously been possible, while at the same time making the resulting tetrafluorethylene material stronger. The rapid stretching also produces a microstructure which is very fine, for example, having a very small effective pore size. When the unsintered extrudate is stretched at a slower rate, either limited stretching occurs because the material breaks, or a weak material is obtained. This weak material has a microstructure that is coarser than materials that are stretched equivalent amounts but at faster rates of stretch.
Densification of an unsintered PTFE extrudate after removal of the lubricant and prior to stretching produces a coarse, highly porous, yet strong, PTFE material which has a microstructure of relatively large nodes interconnected by relatively long fibrils. The desensification step does not change the qualitative interaction of rate of stretch and temperature during stretching that is described above. It merely allows production of coarser articles as compared to prior art articles of comparable strength. Densification can be performed through use of presses, dies or calendering machines.
A water-soluble polymer can be added to a PTFE material after sintering to fill the pore spaces of the material. Also, tearing of porous PTFE tubing in the axial direction can be reduced by coating the tubing with a porous elastomer after sintering the tubing. These processes, however, merely combine a fabricated PTFE material with a non-fluoropolymer material.
Asymmetric porous fluoropolymer materials are defined as porous fluoropolymer materials which have a microstructure that changes in some way from one surface to another. Typically, such asymmetrical materials have a porosity that increases or decreases through the cross-section of the material from one surface to another. One kind of asymmetric PTFE tubing can be produced by heating the outside of a stretched tubular extrudate above the crystalline melting point of the extrudate during the sintering operation while simultaneously heating the inside of the tube to a lower temperature. An asymmetric porous PTFE film can be produced by performing the stretching operation by expanding the film on a pair of rolls having different angular velocities wherein the high speed roll is heated to a temperature higher than the temperature of the low speed roll.
The porous tetrafluoroethylene polymer materials produced by the above-mentioned processes can all be characterized as having microstructures comprised of nodes linked together by fibrils. As discussed above, these nodes and fibrils vary in size depending upon the rate, ratio, and temperature of stretching. The spaces between the nodes and fibrils comprise the pores, and in general, the pore size depends upon the amount the material has been stretched in any one direction. Therefore, as the stretch ratio increases, the length of the fibrils increase and the size of the nodes decrease. Consequently, as the stretch ratio increases, the porosity increases. Furthermore, the materials produced as described above, are all made from an extrudate wholly comprised of only one highly crystalline tetrafluoroethylene polymer resin.